The present invention relates to a method for dry fractional crystallization of fatty substances, including fats and glyceride oils. In particular, the invention relates to the separation of fatty substances in a multi-stage dry fractional crystallization process, in which a high melting fraction obtained in a dry fractional crystallization treatment is recycled to an earlier dry fractional crystallization treatment.
Natural glyceride oils and fats comprise a great many different triglycerides, the physical properties of which to a large extent are determined by the chain lengths and the degrees of unsaturation of the fatty acid moieties. To make natural glyceride oils and fats more suitable for particular applications it is often required to separate them into fractions characterized by fatty acid glyceride distributions which are more homogeneous with respect to the melting behaviour.
For instance, fat blends suitable for producing margarine having a relatively high ratio of poly-unsaturated to saturated fatty acids comprise triglycerides with a specific M.sub.3 /H.sub.2 M (M.sub.3 are triacylglycerols containing saturated fatty acid residues with 12 or 14 carbon atoms exclusively; H.sub.2 M are triacrylglycerols containing saturated fatty acid residues of which two have 16 or 18 carbon atoms and one has 12 or 14 carbon atoms) ratio imparting to margarine good organoleptic properties and suitable consistency at a temperature within the range of 15.degree.-25.degree. C. (see European patent application 89,082).
In the book "Bailey's Industrial Oil and Fat Products" Volume 3, page 5-37 (1985) commercial dry fractionation processes are disclosed in which the oil is cooled to a temperature at which only a higher melting triglyceride fraction crystallizes, followed by separation of the crystallized solids and the liquid fraction, e.g. by filtration or centrifugation.
A multi-stage counter current solvent fractionation process is disclosed in U.S. Pat. No. 2,147,222, in which process a solid phase obtained in a crystallization treatment is passed to the next separation treatment, from which the liquid phase is passed to the former crystallization treatment. This transport of intermediate products is indicated by the term "counter current".
Although solvent fractionation processes involve relatively high capital costs, up to now counter current dry fractionation has not been used on an industrial scale, because conventional separation techniques, such as filtration and centrifugation, possess relatively low separation efficiencies. A high separation,efficiency is required to warrant an effective dry fractionation, because the amounts of the mutually counter current fractions determine the properties and the amounts of the products obtained in the dry fractionation. Finally, counter current dry fractionation is a process more difficult to control because of its complexity.